1. Field of the Invention
This invention relates to a variable magnification zoom finder chiefly used in a compact camera, and in particular, to an inverted Galilean type variable magnification zoom finder.
2. Description of Related Art
An example of an optical system of a conventional virtual image mode variable magnification zoom finder for compact cameras is shown in FIGS. 1A-1C. The optical system of this finder includes a first fixed lens 1, a second fixed lens 2, and a third fixed lens 3 which are arranged on an optical axis a; a moving lens 4 disposed between the first fixed lens 1 and the second fixed lens 2 to be movable along the optical axis a; and a fixed field frame 5 disposed such that it is stationary on the side of a pupil b of the first fixed lens 1.
In the above optical system, a height of a chief ray from the optical axis a at the fixed field frame 5 has a minimum h.sub.1 where the moving lens 4 is located at a wide-angle position (FIG. 1A). As the moving lens 4 is moved from the wide-angle position toward a telephoto position (FIG. 1B), a height h.sub.2 of the chief ray from the optical axis a at the fixed field frame 5 is progressively increased and reaches a maximum h.sub.3 where the moving lens 4 is located at the telephoto position (FIG. 1C). In short, the conventional virtual image mode variable magnification zoom finder is merely designed so that the single fixed field frame 5 is placed to accommodate a change in magnification of the finder.
In general, an angle made by the optical axis with the chief ray emerging from an eyepiece system and reaching a pupil is set to be constant in order to make a field factor constant over the range from the wide-angle position to the telephoto position. In the conventional finder optical system shown in FIGS. 1A-1C, however, a field area is determined by the single field frame 5, and thus the height of the chief ray from the optical axis a at a lens (or a lens unit) is varied in accordance with the magnification change from the wide-angle position to the telephoto position (namely, as shown in FIGS. 1A-1C, h.sub.1 &lt;h.sub.2 &lt;h.sub.3). For this reason, the angle of the chief ray reaching the pupil does not become constant and unwanted light reaches the pupil, causing the problem of hindering the determination of the composition of photography.
In order to avoid this problem, albada type variable magnification zoom finders have been often used in recent years. FIGS. 2A-2C show an example of the albada type variable magnification zoom finder. This albada type finder includes a first fixed lens 6 and a second fixed lens 7 which are arranged on the optical axis a; a half mirror 8 interposed between the first fixed lens 6 and the second fixed lens 7, having a half mirror surface 8a; and a moving lens 9 movable along the optical axis a between the first fixed lens 6 and the half mirror 8.
The albada type variable magnification zoom finder has the advantage that when an optical frame c is attached to the eyepiece system, a photographing area becomes clear and the determination of the composition can be accurately made. However, the albada type variable magnification zoom finder, which imperatively requires the half mirror surface 8a to be situated at an arbitrary place, has the problem that the transmittance of the entire finder system is reduced and as a result, the entire visual field becomes dark, detrimental light such as ghost attributable to the half mirror surface 8a is produced, or a rise in manufacturing cost is caused because of the placement of the half mirror.
Japanese Utility Model Preliminary Publication No. Sho 60-128329 discloses an inverted Galilean type finder in which two field frames of different sizes are provided and alternately placed on the optical axis, thereby allowing the magnification to be changed.
This inverted Galilean type finder, however, requires a complicated mechanism for alternately inserting the two field frames in the optical axis and thus is not necessarily suitable for the latest camera requiring compact design.